I'm having a similar problem as the one referred to in this group post: https://groups.google.com/g/otree/c/kR_fDUWmRLk, only that I haven't tried to create a new MPL variant, but ported an existing one from Otree 3 to Otree 5. I installed the new Otree version, copied the old application over, adjusted one or two settings, and got to testing. The survey/experiment starts well with the first 3 pages or so, so the installation was correct, but eventually I get a crash with this error message (this is 100% reproducible, always at the same point):

"otree.templating.errors.TemplateSyntaxError: Unrecognised filter name 'stringformat:"s"'"

My problem is that the knowledge of how to debug this is no longer available in my department. We were using an inherited setup as-is and wanted to continue to do so on a new server with a more current Otree version. I was just tasked with the porting and I know almost nothing about Otree's inner structure.

Is there anyone here who recognizes this problem and knows how to fix it? If not, where could I gather the necessary knowledge of how Otree works?

Have you solved it? I had the same problem.

stringformat doesn’t exist in otree 5. What reason are you using it for?